Compositional stability of boreal understorey vegetation after overstorey harvesting across a riparian ecotone

Abstract

AbstractQuestionsUnderstanding factors that contribute to the stability of an ecosystem following harvesting is central to predicting responses of boreal ecosystems to increasing human disturbances. While the response of understorey vegetation to harvesting is well understood for upland sites, little is known about compositional stability of riparian understorey vegetation. We examined how compositional stability changes with or without harvesting along an upland to streamside gradient and tested whether compositional stability is affected by pre‐harvest species diversity and composition.LocationLower Foothills sub‐region of the Boreal Plain, ca. 20 km northwest of Whitecourt, Alberta, CA.MethodsWe repeatedly sampled understorey vegetation of four winter‐harvested and four unharvested sites in western Canadian boreal forest. Species covers were measured during the summer prior to harvesting in 2003 (year 0) and in 2004, 2008 and 2010 (1, 5 and 7 yrs after, respectively). We used non‐metric multidimensional scaling to ordinate plots using species covers, and measured vector length in the ordination space to quantify floristic dissimilarity of each plot between year 0 and years 1, 5 and 7, an inverse measure of compositional stability.ResultsFloristic dissimilarity between year 0 and subsequent years was significantly greater in harvested than unharvested sites for all subsequent sampling years. Furthermore, along the upland to stream gradient, floristic dissimilarity was larger on upland than stream sites. Additional analyses revealed that floristic dissimilarity was related negatively to pre‐harvesting species richness and evenness, but positively to bryophyte cover.ConclusionsOur results demonstrate that understorey compositional stability is strongly influenced by harvesting, and streamside communities are more stable than upland communities. Our results indicate that compositional stability of understorey vegetation in response to harvesting is associated positively with pre‐disturbance species richness and evenness, but negatively with bryophyte dominance.